engraving question - slur across notes sung to different syllabes?

[Janek Warchoł]

Hi,

In vocal music slurs usually indicate melismatas. What a slur across
notes sung to different syllabes could mean?
I ask because composer of a piece i'm transcribing urges me to create
such slurs, and doing so is in my opinion contrary to common sense -
and contrary to engraving practice...
The only thing i can come up with is that he wants the notes to be
sung legato, so maybe it would be best to simply write "legato" above
notes?

thanks in advance for any advice,
Janek